1) Field of the Invention
The invention relates to an arrangement for a paper machine, which comprises process elements, and the arrangement includes a measuring beam.
The invention further relates to a blow box which is arrangeable between drying cylinders in a dryer unit of a paper machine to blow air via at least one blow air channel provided in the blow box to guide a moving paper web and/or assist drying.
2) Description of Related Art
In paper manufacture quality properties of paper, such as moisture, thickness and basis weight, are measured as the paper web to be manufactured moves in the paper machine. Paper quality properties are most commonly measured by measuring devices, where measuring sensors are arranged in measuring carriages, which move to and fro in the cross direction of the paper web across its whole width. Since the paper web moves constantly forward, the measuring path of the sensors forms a ‘zigzag’ path, which makes it difficult to distinguish between CD, i.e. cross direction, paper properties and MD, i.e. machine direction, paper properties. Furthermore, generation of a paper cross profile on the basis of the measured properties requires numerous calculations. In addition, generation of the web cross profile is relatively slow because it typically takes several dozens of seconds before the measuring device has once traversed the width of the whole paper web. A measuring device of this kind which moves to and fro in the paper cross direction, i.e. a traversing measuring device, is described in WO 99/44015, for instance.
It is also known to measure the whole cross profile of the paper web by measuring devices, where several measuring sensors are arranged stationary along the whole width of the paper web for simultaneous measuring of the same property of parallel paper web sections. The paper cross profile can be formed very fast by a measuring device of this kind, but the problem associated with it is that a large number of measuring sensors are needed to cover the whole width of the paper web.
There are also prior art solutions where measuring sensors are arranged to move a distance corresponding to part of the paper web width in its cross direction. This is known as a mini-traversing measuring device. This mini-traversing measuring device provides the advantage that the number of adjacent measuring sensors needed to cover the whole width of the paper web is not that large and the paper web cross profile can be formed considerably more accurately and faster than with a measuring device which traverses the whole width of the paper web. An example of a mini-traversing measuring device is described in WO 00/77498.
A problem associated with the existing measuring devices is that in practice they can only be placed at the dry end of the paper machine, typically in the ‘free space’ between the dryer unit and the reeler, where there is enough room for the measuring frames needed to support the measuring devices. In the free space at the dry end the paper web travels unsupported and can thus be measured easily. In that case, however, reliable information is not obtained on the paper web properties at the wet end of the machine, e.g. on moisture at the press section. Consequently, it is considerably slower and more difficult to influence the paper web properties already at the wet end of the machine e.g. by changing the web moisture profile by controlling the press section.
Measuring of the paper web moisture profile immediately after the press section is described in Kosteusprofiilin mittaus paperikoneen märässä päässä (Moisture profile measuring at the wet end of paper machine), Riikka Gerlander, Paperi ja Puu—Paper and Timber Vol. 82/ no. 6/2000. According to the publication, the moisture profile of a paper web is measured by a measuring device which traverses the whole paper web width and is typically arranged to move in a measuring beam which has the same width as the web and is placed between the lower cylinders below the first dryer unit. The problem associated with the solution described in the publication is, however, that there is not enough room for a measuring beam between the lower cylinders below the first dryer unit if there is already a blow box, for example, placed in that space to improve runability of the paper machine and drying of the web. Thus the solution described in the publication prevents placement of the blow box between the lower cylinders, which considerably impairs runability of the machine and drying of the paper web. Furthermore, since measuring is performed immediately after the press section, there are still moisture differences in the z direction (depth) of paper, which causes measurement errors.